Microwave electronic devices, sometimes referred to as radio frequency (RF) devices or vacuum electronic devices (VEDs), are used in systems with important functions such as radar and high speed communications systems, etc. For example, a traveling wave tube is a vacuum electronic device that may be used as an amplifier to increase the gain, power or some other characteristic of an RF signal, that is, of electromagnetic waves typically within a range of around 0.3 GHz to above 300 GHz. An RF signal to be amplified is passed through the device, where it interacts with and is amplified by an electron beam. The electron beam may be generated at the cathode of an electron gun, which is typically heated, for example to about 1000 degrees Celsius. Electrons are emitted from the heated cathode by thermionic emission and are drawn through a cavity or tunnel in the VED to a collector by a high voltage bias, and is typically focused by a magnetic field. If the electron beam directly touches the structure of the VED, it can destroy the VED by overheating and melting the structure.
Magnets are placed around the housing or barrel of the VED, typically along the length of the VED, to focus and steer the electron beam. As illustrated in FIG. 1, magnets (e.g., 10) designed for use around and along a cylindrical VED housing may include a circular cutout 12 to fit around the VED housing, and may be fabricated in pieces 14 and 16 for convenient mounting on the VED housing.
The RF signal enters and exits the VED through ports which can interfere with the magnets. For example, if the RF ports are located on the sides of the housing, they prevent magnets from being placed around the housing at that point. One typical solution is to omit magnets at the RF port locations along the VED housing, but this can allow the electron beam to drift as it passes the RF ports. Another typical solution is the use of a horseshoe magnet 20 with a cutout 22. The cutout 22 allows the horseshoe magnet 20 to slide over the VED housing during assembly, and is aligned with the RF port so that a waveguide or coaxial or other connector can be connected to the RF port at the cutout 22. However, because of the cutout 22 the horseshoe magnet 20 creates an asymmetrical magnetic field which can deflect the electron beam away from the center axis of the beam tunnel in the VED and allow it to approach structures within the VED.